Which imaging modality underestimates urinary stone size, ultrasound (US) or plain‑film KUB?

Neither Modality Underestimates Stone Size—Ultrasound Overestimates

Ultrasound (US) tends to overestimate stone size, particularly for smaller stones (≤5 mm), while KUB does not systematically underestimate or overestimate stone size—it simply fails to detect many stones altogether. 1

Key Evidence on Stone Size Estimation

Ultrasound Performance

According to the 2023 ACR Appropriateness Criteria, stone size estimation at US is limited compared with CT, particularly with smaller (≤5 mm) stones, with a tendency of US to overestimate stone size 1. This overestimation is clinically significant:

• A 2025 study found mean stone size was 8.7 mm on US versus 5.5 mm on CT (p=0.02), with more pronounced overestimation in smaller stones and higher BMI patients 2
• The positive predictive value was only 59% for stones >4 mm by CT, meaning US overestimation may lead to unnecessary surgical interventions in up to 40% of patients with stones >4 mm 2
• A 2005 study demonstrated radiologists significantly underestimated smaller stones on KUB (mean estimated 9.6 mm for an actual 11-mm stone), but this reflects measurement technique issues rather than inherent imaging characteristics 3

KUB Performance

KUB does not systematically misestimate stone size—its primary limitation is detection failure:

• KUB detected only 8% of stones <5 mm** and **78% of stones >5 mm compared to CT 1
• Overall sensitivity is only 29% for stones of any size in any location 1
• When KUB does visualize a stone, size can be measured with rulers or electronic calipers, but 40% of radiologists still use "guestimation" 3

Clinical Implications

The critical distinction is that US actively overestimates stone dimensions (leading to potential overtreatment), while KUB simply misses stones entirely (leading to false-negative results). Neither modality underestimates stone size in a clinically meaningful way.

Common Pitfalls to Avoid

• Do not rely on US measurements alone for surgical planning, especially for stones measuring 4-10 mm on ultrasound—these may be significantly smaller on CT
• US overestimation is worse with higher BMI patients and varies by sonographer skill 2
• KUB cannot be used to rule out stones due to poor sensitivity, but when stones are visible, their measured size is generally accurate

Practical Algorithm

For stone size assessment critical to management decisions:

1. Use non-contrast CT as the reference standard for accurate stone sizing
2. If US shows a stone 4-10 mm and surgical intervention is being considered, obtain confirmatory CT before proceeding
3. KUB is useful for tracking radiopaque stones over time once their presence is confirmed, but not for initial sizing or detection